<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>子元素的高宽</title>
  <style>
  .a1{
    width:100px;
    height:100px;
  }
  .a2{
    width:100%;
    height:100%
  }
  </style>
</head>
<body>
    <div class="a1">
      <div class="a2"></div>
    </div>
</body>
</html>